Panel placement systems serve for delivering positioned panels, e.g. made of aluminum, to a punching press which punches out a predetermined number of blanks from the panel. EP 0 539 669 has made known a panel placement system in which the first gripping means seizes a panel on opposed sides at a transfer position. The panel is moved to the transfer position by second gripping means which engage the rear edge of the panel. The transfer position corresponds to a position of the panel in which the punch performs a first punching stroke on the panel. The feed slide having the second gripping means brings the panel to the transfer position only after the first gripping means have reached their pick-up position along the two axes. The first gripping means are adjustable along two orthogonal axes to allow an offset pitch to be chosen for the surfaces to be punched out, for a favorable material utilization. As was mentioned the transfer of the panel to the first gripping means at the transfer position takes place during the first punching step during which the panel is fixed by the punching die. After the machining of the preceding panel ends the two axes of the feed for the first gripping means and the feed slide travel to the initial and pick-up position in which a new panel is picked up. The panel placement system described helps to significantly reduce the transfer time for the panels with the press continuing to run. Nevertheless, there is at least one idle stroke between the punching step of a preceding panel and the first punching step of the succeeding panel. The idle stroke could possibly be avoided by lowering the die ring. However, the benefit which is achievable is brought about at the expense of positioning errors as the panel is pulled down during the last cut.
During the last stroke of the known placement system, one of the two pairs of feed pliers is directly at one of the outer dies. If it is to be ensured that the upper edge of the lower gripper mouth defines a plane with the die ring to prevent the panel from being pulled down during the last stroke the pliers are in a specifically formed pocket of the lower die ring when cutting diameters are small (<100 mm). The panel which is newly to be introduced has to be passed through under the feed pliers. However, this can be accomplished only after the pliers are moved back a sufficient length to leave enough space for the panel which is moving in. The front edge of the panel which is newly to be introduced has to be raised to the level of the die ring. Even if pockets do not require to be provided in the die rings when cutting diameters are larger it is then difficult for thicker metal sheets to get through between the pliers and the die ring. The gap between the pliers and the die ring can also be released by raising the pliers. However, this procedure takes time. Also, a panel placement system is known in which the gripping pliers engage the rear panel edge. Also in this configuration, however, the gripping pliers will be at a position identical to a die ring as in the previously described system during the last stroke. A more precise look during the engagement effected on the rear edge reveals, however, that if there is a diagonal feed only the pushing pliers act during a change of row. This results in a start of acceleration only at certain points and causes a diagonal formation of waves which takes a very long time to pass through the entire panel. Not until the panel lies in a flat condition again is precise punching is possible. A dynamic operation is no longer possible from the moment at which the undulary motion begins. The problem specifically occurs at the start of panel machining when the panel still has its full size and the distance of the pliers from the die is minimal.
The punching press runs so fast as is allowed by this feed motion. Then, however, no time will be left for the panel to undergo additional handling such as lifting or carrying away. Thus, this lack of time results in a reduction of the possible number of revolutions of the press and, hence, a reduction of the performance of the entire system. This same applies to all panel feed systems which are known.
As mentioned already, the known gripping means are formed by pliers. The pliers wear rapidly, primarily when panels of chromium-plated sheet steel are employed. The chromium will crumble off the panel edges and causes debris to work into the mouths of the pliers. Wear also takes place in the joints of the pliers. Since the plier size has to be small there is a need to provide bearings of a simple construction only, which further increases wear.
When punching is done in the final row the stability of the remaining grid will usually collapse because the residual sheet metal pieces are only left suspended in webs and, therefore, are still joined to the gripping pliers only to a limited extent. This causes dimensional variations with respect to the position of the panel in the die.